1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image, which is used to develop an electrostatic image, followed by heat fixing, in an image forming process such as electrophotography.
2. Related Background Art
There is an image forming method in which an electrical or magnetic latent image on a recording member is converted to a visible image by attracting to the latent image, electroconductive or magnetosensitive fine particles called a toner.
In electrophotography, which is a typical example thereof, a large number of methods have been conventionally known, as disclosed, for example, in U.S. Pat. No. 2,297,691. In general, in such electrophotography method, an electrostatic latent image is formed on a photosensitive member, utilizing a photoconductive material and according to various means, and subsequently the latent image is developed using a toner to form a toner image. Then the toner image is transferred to a transfer medium such as paper if necessary, and the toner image is fixed to the transfer medium by the action of heat, pressure and/or solvent vapor. A copy is thus obtained. At present, a fixing method that utilizes heat is prevaling in view of its advantages in fixing strength of copies, readiness to handle transferred objects and ease in operation. Such a fixing method includes a method that utilizes radiation heat as in the heat chamber system, and also what is called heat roller fixing system in which a heated roll type heating member is pressed against a toner image to fix the image. The latter system is employed in most machines in view of its high heat efficiency, high-speed adaptability and high safety. However, in spite of the high heat efficiency, the energy used in heat melting occupies a reasonably large proportion in a copying machine. In addition, there is a disadvantage that it is difficult to avoid what is called the offset phenomenon wherein the toner adheres to a heat roll because of direct contact with a molten toner image to soil subsequent images and in an extreme case what is called the wind-around phenomenon which the whole medium to which the toner image is fixed is wound around a heat roll. In order to decrease the energy required for the melting of toner, it can be greatly effective to increase the quantity of components capable of melting at a low temperature, and in order to lessen the adhesion of toner to a heat roller, to incorporate in the toner a wax or oil that does not melt together with a binder resin of the toner and becomes fluid faster than, and has a smaller cohesive energy than, the binder resin of the toner. Such materials, however, are disadvantageous in that they may at the same time decrease the fluidity of toner and very much lower the developing performance.
Toners used for such purpose have been hitherto usually obtained by mixing and melting in a thermoplastic resin a coloring material comprised of a dye and/or a pigment and a magnetic material and uniformly dispersing the coloring material, followed by pulverizerion and classification to produce a toner having the desired particle diameter. This method is relatively stable as a technique and can enjoy relatively easy control of the materials and process. However, because of exposure of contents to rupture cross-sections, it has been impossible for the aforesaid component for giving a low melting point and component for giving release properties to be incorporated in quantities large enough to be effective. Besides, this method has a poor energy efficiency since the materials are once melted together with a binder resin so that they are mixed and made stationary, and further the molten product is cooled, followed by mechanical pulverization. Moreover, the toner tends to have a broad particle size since its particles are finely divided by mechanical pulverization, so that the toner must be managed in the subsequent step of classification to have the desired particle size distribution. This may bring about a difficulty that the products can not be obtained in a higher yield. In order to solve such problems, a process in which the toner is produced by what is called suspension polymerization is proposed as a new production process.
For example, Japanese Patent Publications No. 36-10231, No. 47-51830 and No. 51-14895 and Japanese Patent Applications Laid-open No. 53-17735, No. 53-17736 and No. 53-17737 disclose a process for producing a toner by the suspension polymerization. In the suspension polymerization, materials that are required to be contained in a toner as exemplified by a binder resin, a colorant such as a dye or a pigment, a magnetic material, carbon black, a charge control agent and a release agent such as wax or silicone oil are uniformly dissolved or dispersed in polymerizable monomers optionally together with a polymerization initiator and a dispersant to form a polymerizable composition, and this polymerizable composition is put in an aqueous continuous phase containing a dispersion stabilizer to form fine particles by the use of a dispersion machine, followed by polymerization reaction to effect solidification so that toner particles with the desired particle diameters can be obtained in one step when the polymerization is completed.
This suspension polymerization, which requires no pulverization step, may make it possible to omit not only the melting step and pulverization step but also the subsequent classification step, and can be greatly effective for cost reduction such as energy saving, time shortening and improvement in process yield.
The present inventors have hitherto developed a polymerization toner in which silicone oil, a wax or a low-molecular weight component with a molecular weight of not more than 3,000 has been incorporated in a large quantity, which otherwise can not be produced or stored if produced by the usual method relying on kneading and pulverization. This polymerization toner is produced utilizing the properties that polar components are localized in the vicinity of particle surfaces and non-polar components are concentrated to the centers when suspension polymerization is carried out in an aqueous medium. Thus, a toner capable of being fixed at a low temperature and requiring no application of a release agent to a fixing assembly during fixing has been obtained.
In the suspension polymerization, in the case of styrene-acrylic vinyl type polymerizable monomers, a toner composition that can be used as a heat-fixing toner on the whole can be obtained when a polymerization initiator is used in an amount of from 0.5% to 20% by weight and the polymerization temperature is so set that the half-life period of the polymerization initiator is controlled to be from 0.5 hour to 30 hours.
Even when the polymerization conversion is at least 90% under such conditions, toner particles tend not to coalesce into a rice cake, even when stirring was stopped. For example, at the moment when the polymerization conversion has reached 97 to 98%, toner particles may be taken out and dried, so that they can be used as a toner without any particular problems.
However, in the case when a low-temperature melting wax is contained in this polymerization toner system in a large quantity, though images with a good-quality can be obtained without any problem in a normal environment, a lowering of blocking resistance and a lowering of developing performance have occurred after the toner has been left in an environment of a high temperature.
U.S. Pat. No. 4,971,879 discloses a toner resin obtained by suspension polymerization, having therein not more than 200 ppm of remaining monomers.
This U.S. Pat. No. 4,971,879 discloses decreasing the quantity of monomers remaining in a toner resin (a resin used for a toner), which is fundamentally different from the technique concerning the decreasing of remaining monomers in the toner obtained by suspension polymerization, containing the above wax in a large quantity.
Besides, taking note of the shape of toner particles obtained by suspension polymerization, they are truly spherical. Such a shape has been hitherto deemed to be suitable for achieving a high image quality. A toner with spherical particles, however, tends to cause a deterioration of its performance when various external additives are used, and can not be a toner having an excellent running performance. The toner with spherical particles also has so strong an adhesion to a photosensitive member that it tends to cause an image deterioration accompanied by faulty transfer and also can cause faulty cleaning after the transfer step. Such difficulties have been confirmed.